1. Field of the Invention
The present invention relates to a droplet ejecting head having a nozzle hole for ejecting a droplet of a liquid or the like therethrough. Such a droplet ejecting head may be an inkjet printhead, but not limited thereto. For instance, there are also a head which ejects droplets of a material having an electrical conductivity onto a substrate to form wiring or a circuit on the substrate, and a head which ejects droplets of a solidifiable material to form a three-dimensional shape which is then solidified to obtain a three-dimensional body.
2. Description of the Related Art
In such a kind of droplet ejecting head, the size and shape of the droplets are desired to be regulated with high accuracy and precision. This will be described more specifically by holding up the inkjet printhead as an example.
Disclosed in Japanese Patent Application Laid-Open No. 2001-246744, there is known an inkjet printhead comprising a cavity unit formed of a laminate of a plurality of plates including a nozzle plate, a piezoelectric actuator, and a flexible flat cable. The cavity unit has a nozzle surface as an external surface of the nozzle plate through which are formed a plurality of nozzle holes to be opposed to a recording medium, and a plurality of pressure chambers respectively corresponding to the nozzle holes. The piezoelectric actuator is superposed on a surface of the cavity unit opposite to the nozzle surface, so as to selectively pressurize the pressure chambers to eject an ink droplet from a nozzle hole corresponding to the pressurized pressure chamber. The flexible flat cable is for supplying electric signals therethrough to the piezoelectric actuator.
In such an inkjet printhead, the piezoelectric actuator selectively reduces the volume of the pressure chambers in accordance with the signals received through the flexible flat cable, so as to eject ink droplets through the nozzle holes onto the recording medium. Therefore, the shape of the nozzle holes considerably affects the ink ejection performance.
It is known that in order to ensure a good ink ejection performance, the nozzle hole is suitably formed in the shape such that a diameter of the nozzle hole is smaller on an ink jetting side, namely, at the nozzle surface, than on an ink inflow side, namely, at the surface of the nozzle plate opposite to the nozzle surface. That is, the nozzle hole is tapered or narrowed from the internal side toward the external, jetting side. For instance, Japanese Patent Application Laid-Open No. 6-246917 (see FIGS. 3 and 4) teaches to stabilize the ink ejection performance by configuring the nozzle hole to have an appropriate ratio of an area of its open end on the ink jetting side to an area of its open end on the ink inflow side, and an appropriate angle of the taper.
For instance, the nozzle holes having such a tapering shape may be formed by irradiation with a laser beam. More specifically, a plate material to be the nozzle plate is perforated with a laser beam, namely, the side of the plate material to be the surface opposite to the nozzle surface, or the surface on the ink inflow side, is irradiated with the laser beam. In this laser beam machining, the properties of the laser beam make each of through-holes or the nozzle holes tapered down or gradually narrowed in the direction of thickness of the plate material from the irradiated side, with the diameter of the nozzle hole gradually decreasing in the same direction to have an intended value at the open end on the ink jetting side, with a relatively high precision. Even when the diameters at the opposite open ends and the tapering shape of the nozzle hole are appropriately specified, however, it is difficult to obtain the nozzle holes of the shape precisely as designed. This is because of that in actual laser beam machining the laser beam can not be accurately focused at a point, and results in a rounded intersection of an inner circumferential surface of the nozzle hole and a plane surface of the nozzle plate on the irradiated side. That is, an edge of the open end of the nozzle hole on the ink inflow side is rounded. In addition, since it is difficult to position with high accuracy and precision the plate material to be the nozzle plate relative to the focal point of the laser beam, the degree of rounding or chamfering varies, in turn making it difficult to form the nozzle holes in a desired shape with high accuracy and precision.
The nozzle holes formed by the laser beam machining are subjected to a screening inspection whether the nozzle holes are good or bad. Conventionally, the open ends on the ink jetting side are strictly controlled or inspected since the shape of the open ends on this side significantly affects the ink ejection performance and the open ends on the opposite or ink inflow side are less strictly controlled.
To further improve the stability in ink ejection performance of the inkjet printhead, however, it is required to optimize the shape of the open ends at the ink inflow side also, in view of the minute variation in shape there with the above-mentioned rounding, in addition to the conventionally implemented controls including that on the ratio of the open areas.